It is well known to provide metallic substrates with metallic protective coatings if those substrates are likely to encounter hostile conditions which could be detrimental to the properties of the basic substrate material. Such protective coatings may for instance be applied by the techniques commonly known as flame or plasma spraying. In those techniques, particulate alloys entrained in a hot gaseous or plasma environment are directed on to the substrate to be coated. The particles are in a molten or semi-molten state as they hit the substrate surface whereupon they cool and solidify to form a bond with the substrate. The bonding between the particles and the substrate surface is essentially mechanical in nature and consequently prone to failure if for instance impact or rubbing forces are encountered by the coating.
Improved bonding between metallic coatings and metallic substrates may be achieved by applying the coating to the substrate in rod form whilst simultaneously melting the application area with a high intensity heat source such as oxy-acetylene flame or a plasma or argon shielded arc. Since both the coating material and substrates are melted, a certain degree of mixing takes place between them so that the resultant bond is usually very strong. However the technique is difficult to control accurately and consequently coatings of variable consistency and depth are likely to result.
An alternative method of improving bonding between metallic coatings and metallic substrates comprises applying the coating material to the substrate in powder form so as to provide a layer of the powder which is subsequently melted using a laser beam. Whilst improved bonding results, it is difficult to accurately control the coating area and depth.